When a patient is diagnosed with an abnormal cardiac rhythm, one form of therapy often applied is the use of an implantable medical device to restore normal rhythm to the heart. In case of a slow heart rate, a pacemaker is often used to deliver synchronized pulse stimuli to restore normal rhythm to the heart. If the heart rate is less than 60 beats per minute ("bpm"), the condition is known as bradycardia, and the delivery of the stimulation pulses is referred to as pacing. A variety of pacemakers are available to treat this condition by artificially stimulating myocardial tissues using electrical pulse stimuli. Conventional pacemakers known in the art typically include a built-in sensor circuit for sensing, through attached sensing electrodes, the rate at which the heart is beating. The fundamental way the pacemaker identifies the presence of a bradycardia is by detecting that the heart rate has reached below a critical value. Since most episodes of a bradycardia exhibit a rate less than 60 bpm, the pacemaker is typically programmed to apply therapeutic pacing pulses when this rate is reached.
In case of a fast heart rate, however, an Implantable Cardioverter-Defibrillator ("ICD") device is often used to deliver high energy stimulation pulse or pulses to restore normal rhythm to the heart. If the heart rate is greater than 150 bpm, the condition is known as tachycardia, and the delivery of the stimulation pulses is referred to as cardioversion. If the heart rate is greater than 240 bpm, then the condition is known as fibrillation. The condition is characterized by rapid and unsynchronized quivering of the atria or ventricles. If ventricular fibrillation ("VF") is involved, then the ICD device delivers higher energy stimulation pulses (referred to as "defibrillation") to recover the heart from its fibrillation state.
Conventional ICD devices known in the art typically include a built-in sensor circuit for sensing, through attached sensing electrodes, the rate at which the heart is beating. The fundamental way the ICD identifies the presence of a sustained ventricular tachycardia ("VT") is by detecting that the heart rate has exceeded a critical value. Since most episodes of sustained VT exhibit a rate in excess of 150 beats per minute ("bpm"), the ICD device is typically programmed to initiate a cardioversion therapy when this rate is reached. The type of therapy to be used depends on the device as well as the hemodynamic status of the patient during the tachyarrhythmia episode. For antitachycardia pacing, additional detection criteria may be programmed to enhance the certainty that a VT is present rather than a supraventricular tachyarrhythmia (e.g., atrial fibrillation). Such detection criteria are well known in the art and include the identification of cycle length stability, the abruptness of onset of the tachyarrhythmia, and the duration of a sustained rate. If the sensed heart rate is higher than a rate threshold of around 240 bpm, then a VF is often the sensed event. The ICD is then triggered to deliver a defibrillation pulse to the heart through the stimulating electrodes.
Typically, the physician who is implanting the pacemaker and/or ICD device determines the correct rate thresholds, which are needed for the particular patient. In programming the pacemaker/ICD device, however, the rate thresholds may not necessarily be appropriate for the particular patient. In addition, due to a coupled drug therapy, the patient therapeutic levels may often vary over time. When the patient is not being monitored, the physician will not be able to identify the effect of the selected therapy levels. Consequently, the physician cannot monitor the operation of the pacemaker/ICD device and, particularly, the patient's heart response to the applied therapy. Therefore, there is a need to enable a physician to better diagnose a patient's particular need for a cardiac therapy. The physician should determine a patient's progress in connection with and response to the applied therapy.